1. Field of the Invention
This invention resides in the field of NOx emissions from combustion facilities, and methods and plant equipment using selective catalytic reduction (SCR) for controlling and reducing the emissions.
2. Description of the Prior Art
A widely used process for the reduction of NOx emissions from coal-fired utility boilers and in combustion flue gases in general is the process known as selective catalytic NOx reduction (SCR). In this process, the NOx in the flue gas is reacted with oxygen and ammonia over a solid catalyst which is either a metal such as titanium, vanadium or platinum, or a zeolite or a ceramic, to reduce the nitrogen in NOx to molecular nitrogen with water as a by-product. The utility industry has invested billions of dollars in SCR units, and these units collectively utilize a volume of catalyst that is on the order of 200,000 m2.
The performance of an SCR unit, as measured by the change in NOx concentration across the unit (ΔNOx), depends on the catalyst activity and the area velocity Av of the unit according to the relationΔNOx=1−e−K/Av  (1)
where K is the catalyst activity and Av is defined by the relation
                              A          v                =                  Q                      VA            s                                              (        2        )            
where Q is the flue gas flow rate through the unit, V is the bulk catalyst volume, and As is the surface area of the catalyst per unit volume of the catalyst. Contributing to the catalyst activity K are such factors as the mass transfer coefficient, the kinetic rate constant and various geometric factors. The typical SCR catalyst is rated by its vendor with an initial catalyst activity Ko. The catalyst activity and hence the NOx reduction potential of the unit decrease gradually over time as the catalyst is subjected to macro- and micropore diffusion, absorption, desorption, chemical reactions involving the catalyst itself, and catalyst poisons. To maintain the desired level of NOx reduction, the rate of ammonia injection must then be increased to compensate for the decrease in catalyst activity. This in turn results in greater amounts of unreacted ammonia leaving the unit (the “ammonia slip”) and therefore greater cost in operating the unit, a greater risk of pollution, and possible adverse impacts on downstream equipment. Catalyst degradation is further complicated by the fact that in large catalytic reactors the catalyst is deployed in two or more distinct and separated layers, with different layers tending to degrade at different rates. Even in a single layer, the catalyst can undergo different degradation rates at different locations in the layer. As the catalyst continues to degrade, replacement or regeneration is eventually necessary. Typically, one-third to one-fourth of the catalyst is replaced or regenerated approximately every 15,000 to 25,000 hours of continuous use.
A well-run boiler or combustion facility will have a catalyst management procedure for the SCR unit that will allow the facility to comply with the regulatory requirements for NOx and NH3 emissions, and yet conform to the outage schedule for the facility. Periodic monitoring of the catalyst provides the most efficient use of the catalyst and allows operators to maintain the facility in compliance with the regulations. Monitoring methods that are in current use introduce inefficiencies of their own, however.
One of these methods is by monitoring the ammonia slip. Another is by monitoring the ammonia content of the fly ash. Either method provides only an indirect indication of the catalyst activity, and only a gross or overall indication of any loss in activity. These methods will not differentiate between situations in which all catalyst layers are losing activity at approximately the same rate from those in which upstream layers are losing activity at a greater rate than those downstream. A further difficulty is that increases in the ammonia slip or in the ammonia content of the fly ash may be the result of factors other than catalyst activity, such as an ammonia injection grid that is not properly adjusted or any other irregularity in the ammonia injection system.
Another monitoring method is that in which samples of catalyst are removed from the reactor and transferred to a laboratory for direct determinations of the catalyst activity or the activity ratio K/Ko. This can also be done with catalyst coupons retained in the reactor in a special removable holder. Removal of the samples or coupons however usually requires that the unit be taken off-line. Unless the unit is expressly shut down for the sampling, the time interval between sampling opportunities will be dictated by the outage schedule of the unit rather than concerns over the catalyst activity and can be very long. For those units operating on a schedule that is designed to accommodate the ozone season, for example, outages may occur as seldom as once or twice a year. For units operating year-round, the sampling frequency may be even lower, such as once every other year. A further problem with the withdrawal of catalyst samples is that the analyses of these samples provide no information regarding how any observed decrease in catalyst activity occurred, i.e., whether the decrease occurred slowly and gradually over time or by a step change resulting from a boiler upset, a fuel change, or some other occurrence not related to the SCR unit itself.